Light can be controlled using standard lenses and mirrors. These passive methods can be made active via mechanical motion. For example, mirrors can be placed on motorized stages or piezo mounts to move a mirror to control either the direction of light propagation and/or the total optical path length of a system. By altering the physical path length, the optical phase delay (OPD) of the light may be controlled. This technique is used in Fourier transform spectrometers, external cavity tunable diode lasers, tunable filters, etc.
However, mechanical control over light is undesirable for several reasons. First, it is difficult to make such mechanical devices compact. Second, the mechanical nature of such moving devices have limited lifetimes due to mechanical wear and failure issues. Third, mechanical devices are inherently vibration sensitive, which limits the type of environment in which they can be used. Finally, mechanical devices necessitate a level of design complexity including gears, bearings, and other mechanical components, which add cost, expense, and maintenance issues to such designs.
Rather than move a lens or a mirror with a motor or actuator, light can be controlled through the use of waveguides. For instance, U.S. Pat. No. 5,347,377 entitled “Planar Waveguide Liquid Crystal Variable Retarder” relates generally to providing an improved waveguide liquid crystal optical device, and discloses in Table I the use of alternating current voltages between 2 and 50 volts rms. This patent teaches, among other things, controlling the optical phase delay for TM polarized light.
Light can be characterized as having various polarized components, such as TM (transverse magnetic) and TE (transverse electric) polarizations, which relate to the magnetic and electric field components of light. Generally, if one chooses a reference plane that is oriented perpendicular to the light propagation direction, TM polarized light means that the magnetic field of a light wave is traversing or parallel to that plane, while the electric field of the light is substantially perpendicular to the plane. TE polarized light is characterized by the electric field of the light traversing or parallel to the same plane, while the magnetic field of the light is substantially perpendicular to that same plane.
In another non-mechanical technique for controlling light, thermo-optics can be used to control light. The temperature of a waveguide, constructed with thermo-optic material such as silicon oxynitride, can be used to alter the index of refraction (n) for light traveling through the waveguide. Such thermo-optic approaches typically provide for only limited changes in index of refraction (dn/dt≈1.5×10−5/° C.), which in turn necessitates large temperature changes (up to 500° C. or higher) for significant light control. These devices can be power consumptive, which may be prohibitive for many applications.
An electro-optic approach may be generally less power consumptive, than thermo-optics. With conventional waveguides, electro-optic materials, such as LiNbO3, are employed whereby a voltage applied across such material changes the index of refraction, n. However, with these conventional techniques, the index of refraction can only be changed a very small amount, such as 0.0001 per kilo volt for bulk materials such as LiNbO3. This limitation makes this type of light control extremely limited due to the high amount of voltage needed for significant light control.
While liquid crystal optics have become widespread in display applications, in such applications light is attenuated but the optical phase delay is not appreciably altered, or only to a very small degree, typically less than one wavelength of light (<1 micron). Use of conventional liquid crystal optical techniques to achieve active optical control would generally require prohibitively thick layers of liquid crystal (>100 microns), which would render the device highly opaque and slow. The thick layers of liquid crystal may be difficult or impossible to control. Furthermore, liquid crystal displays are typically polarization dependent.
Accordingly, as recognized by the present inventors, what is needed is a liquid crystal waveguide for controlling light that permits active control of the propagation of several different polarizations of light such as TM and TE polarized light through the waveguide in a manner that provides for low losses.
It is against this background that various embodiments of the present invention were developed.